Anodic treatment of magnesium and its alloys to form protective coatings thereon



Patented Aug. 26, 1947 ANODIC TREATMENT OF MAGNESIUM AND ITS ALLOYS TO FORM PROTECTIVE COATINGS THEREON Herbert Waterman, Los Angeles, Calif, assignor toTurco Products, Inc., Los Angeles, Calif., a corporation of California No Drawing. Application April'23, 1943, Serial No. 484,218

9 Claims. 1

This invention has to do with the formation of protective coatings on magnesium and magnesium alloys by electrolytic treatment and relates more particularly to anodic treatment and electrolytic baths for the purpose.

Although magnesium and its alloys have become increasingly recognized in recent years as having peculiar advantages in industry, a difficulty in their use has been the lack of suitable methods of providing a really dependable protective coating on the surface of the metal to safeguard against corrosion and to provide a permanent base for paint. Many such methods were proposed which may be classified under three Another object is the provision of an improved electrochemical bath in which the coating is produced.

Still further objects and corresponding advantages are inherent in my invention and those will appear obvious from the following detailed explanation of the presently preferred procedure.

In a preferred practice of my invention I use an electrolyte made up of a solution of soluble carbonate, using relatively pure sodium carbonate in quantities sufficient to saturate the solution at its boiling point. This is best accomplished by using sodium carbonate monohydrate in a quantity sufficiently large that a substantial part headings: paint applications directly over the of it remains on the bottom of the vessel containmetal surface, chemical dip treatments, and elecin the bathn t manner it is rt in that trochemical treatments, or combinations of'those the solution remains saturated a all times a d methods. A chemical dip treatment followed by temperetllles- It is preferable t use distilled n application Of paint, coating h b e d eme water in the bath as I find that this provides the the best procedure heretofore, 29 best and most dependable coating. Tap water The service performance of surface protection of normal P y y b used, although the qua of magnesium and its alloys provided by such ity of the coatings may then vary to some extent. methods has been erratic and unreliable, and has The term magnesium as herein used means left much t b d i They provide Way any article made of pure magnesium or its comto tell in advance whether any coating on any mefeiel y particular piece of magnesium or its alloy will The magnesium article to be Coated is then perfgrm adequately in service T Widespread immersed in the electrolyte while connected to use of magnesium in t aeromane and other the positive pole of a direct current source, the industries is retarded to a large extent by this material thus being the anode. A commercial unce t int 30 110 125 volt direct current line may be used as According to the present practices in this art the v a o ur th p at f pla there is no certainty that the coated magnesium ing 0 immersing the magnesium article in the article possesses a really dependable coating, and electrolyte, t current i preferably k pt nthere is no reliable method of determining from stem? at Whatever Value the available equipment t cgating operatig-n whether the article is of will furnish. It is preferable to use the highest proper quality, r wh th it t i segregation current value possible consistent with the means of alloying constituents or whether the surface provided for maintaining the electrolyte t 0 area contains foreign elements which will render heel its boiling temperaturep in the coating poor. Thus, in the past, many failures my preferred practice the initial C e t alue of the coated material have resulted which have 40 is approximately 100 amperes per Square o been 'i nexplainable, although an initial current value range of 50-120 By my invention I aim to Overcome those past amperes per square foot may be used. If initial uncertainties and shortcomings, particularly incurrent value is substantially below 100 p es sofar as electrochemical treatment is concerned, the time required for Completing the co ting and to provide a method of and electrolyte for Operation is illereased- If e y, a c nvenel'ectrolytic treatment of magnesium and its alloys lent means of regulating the initial Current which is controllable, fully dependable, enables Siiy is use Of a Series resistance 0 Of a 0011- uniform and implified shop practices t produce stant current generator. If the magnesium articoatings of definitely determinable character, and (lie is immersed hefere application of t t, which does not require any elaborate control h t Of Coating y generally b cr asedfacilities, Prior to the treatment, it is desirable, although Another object is the :production of a dense not essential, to clean the surface of the magnecoating which is self sealin so that, upon its sium of any dirt, grease or other undesiredcoatcompletion, it will providean adequate protection ing, by any of the Well-known means. The elecfor thesurface of magnesium and its alloys. trolyte which I use is in itself an alkaline cleaner and is capable of handling reasonably dirty objects without prior cleaning. Such procedure is enerally undesirable, however, since the coatings produced may not be at times as perfect a they Would be if the object had been previously cleaned, depending upon the material and extent of surface contamination. Also, in such case the electrolyte life will be somewhat impaired.

The coating begins to form as soon as the magnesium object is immersed inthe electrolyte. This coating presents a relatively high electrical resistance when properly formed and thus causes a steady gradual decrease in the electric current which may flow at any given voltage. For example, using the voltage sufiicient to provide an initial current density of approximately 100 amperes per square foot, if the coating formed is a proper one, the density will decrease to about 0.2 ampere per square foot within ten minutes, approaching ultimately some small fixed asymtotic value which varies with the purity of the alloy and is generally under 0.04 ampere pe square foot for the usual commercial magnesium alloys used.

If the electrolyte is contaminated, or if the magnesium being treated contains flux inclusions, or segregation of alloying constituents, or if the surface is of low quality due to the presence of foreign elements on which no magnesium carbonate coating may be produced, the coating will be relatively porous or the surface will become etched rather than coated, so that the functioning of the coating as an electrical resistance is poor or erratic. Consequently, the current density will decrease for a short while during the early stages but will then tend to go up again. Where the magnesium is of poor quality, I have found it to be impossible to reduce the current density below time of its initial value, and at times, if the magnesium is of very poor quality, it is impossible to reduce the current density to below Te of its initial value. Such objects are defective and should be discarded.

While, if the electrolyte is materially contaminated it will aiTect the decrease in current density, it is relatively easy to keep the bath free of any serious contamination and therefore it is a simple matter with the use of my invention to determine by measuring the current density from a given voltage just what are the characteristics of the coating and whether the magnesium object is faulty.

The coating produced by my invention is relatively non-porous as compared with the relatively porous coatings on magnesium resulting from electrochemical treatments heretofore attempted. This improved type of coating appears to be due, at least to some extent, to the fact that my coating has a greater volume than that occupied by the magnesium used in forming the coating, While the previously known coatings were relatively smaller in volume. My coating is relatively dense and is impermeable to the usual corrosion media to which magnesium is generally exposed.

Another factor contributing to the improved results of my method appears to be that my method produces an anhydrous magnesium carbonate coating which has no appreciable solubility in salt water saturated with CO2 or other normal corrosive, while the usual form of magnesium carbonate produced in aqueous media is relatively hydrated and has substantially the formula MgCO3.3H2O. v

The duration of the treatment determines the thickness of the coating. I find sixty minutes to be adequate in most cases and after such period of treatment using a magnesium object of proper quality, the current density will have dropped to as low as 0.04 ampere per square foot. If such current density drop is not obtained, it denotes that the coating is not of the proper quality. The temperature of the bath should be maintained above 50 C., the optimum being C,

While in my referred example of bath I prescribe a saturated solution of sodium carbonate, I wish it understood that, in its broader aspects, my invention contemplates the use of any alkali carbonate capable of forming an anhydrous magnesium carbonate coating on magnesium and its alloys. For instance, ammonia carbonate may be used and other concentrations may be used. However, for any specific concentration an appropriate voltage and current schedule should be adopted, based upon the optimums which I have given hereinabove for a saturated sodium carbonate solution.

I claim:

1. The method of electrolytically forming a protective film on a magnesium object which comprises electrochemically treating the object, as an anode, in an electrolyte consisting of a substantially saturated solution of alkali carbonate while maintaining the solution about 50 C., the anodic treatment being continued until the current flow decreases to a relatively small value due to the electrical resistance of the resulting film.

2. The method of claim 1, wherein said alkali carbonate is sodium carbonate,

3. The method of claim 1, wherein said relatively small value of current flow is a current density below 0.2 amperes per square foot.

l. The method of electrolytically forming a protective film on a magnesium object which comprises electrochemically treating the object, as an anode, in an electrolyte consisting of a substantially saturated solution of sodium carbonate while maintaining the solution above about 50 C. with sodium carbonate in the solid phase in contact with said saturated solution, the anodic treatment being continued until the current flow decreases to a relatively small value due to the electrical resistance of the resulting protective film.

5. A method of treating magnesium articles to form a protective coating thereon which comprises subjecting the article, as an anode, to electrolytic treatment in an electrolyte consisting of a substantially saturated solution of alkali carbonate at a temperature above about 50 C. while imposing a, voltage sufficient to maintain on the article a current density at the beginning of the formation of the coating of above 50 amperes per square foot and continuing the anodic action until the current flow decreases to a relatively small value, while determining the chaarcteristics of the coatingbeing formed by measuring the rate of decrease in current density caused by the formation of the coating and discarding the articles so treated which give erratic current density and which never attain coatings with predetermined electrical resistance.

6. The method of claim 5, in which said current above 50 amperes is a current density of between 50 and amperes per square foot.

'7. The method of claim 5, in which said alkali carbonate is sodium carbonate.

8. The method of claim 5, in which alkali carbonate in the solid phase is maintained in contact with the electrolyte used in the treatment.

HERBERT WATERMAN.

REFERENCES CITED The following references are of record in the file of this patent:

r UNITED STATES PATENTS Number Name Date 1,117,240 Presser Nov. 17, 1914 2,146,029 Schimkus Feb. 7, 1939 10 2,197,611 Fischer et a1 Apr. 16, 1940 OTHER REFERENCES Trans. Electrochem. $00., Vol. 68 (1935), pp. 533-547. 

